How to scale a mesh without destroying the UV Map
When you scale your object on Z axis in 3D viewport, you must be careful to adjust your geometry in UV Image Editor because it don't have Z axis and you have to adjust on both X and Y axis.

You must scale the UV Unwrapped Geometry like this:

See images and scale accordingly.

Default Mesh with texture.

Scaled on Z Axis with Texture


Can you see I have scaled the UV Map differently in both images.
Shortcuts are same for scaling in UV Image Editor that is S


  There are many free tutorials out their who covers Texture Mapping.


All the Best.